Since insects vector many diseases of humans and animals, understanding the interactions between parasites and their insect hosts is an important area of study. The insect immune system plays an essential role in defending insects against invading organisms, and in large measure defines whether a given species is a suitable host for a given parasite or pathogen. Insect blood cells (hemocytes) often eliminate unicellular pathogens like bacteria by phagocytosis while multicellular, metazoan parasites are killed by encapsulation. Reciprocally, some pathogens and parasites avoid elimination by disrupting hemocyte function. Our previous studies revealed that the parasite Microplitis demolitor disrupts the encapsulation response of its host, Pseudoplusia includens, through the actions of a symbiotic virus called M. demolitor polydnavirus (MdPDV). MdPDV induces the apoptosis of one class of hemocytes in P. includens while rendering another class of blood cells incapable of adhering to the parasite. Based on these observations, we propose to identify the viral genes responsible for inducing these alterations in host hemocytes, and to characterize how the host immune system responds to viral infection. Specific aims for this proposal are to characterize: 1) the MdPDV genes involved in induction of apoptosis and disruption of hemocyte adhesion; 2) how MdPDV interacts with the apoptotic pathway of host cells; 3) how MdPDV affects mediators of hemocyte adhesion, and; 4) how host hemocyte populations respond to parasitism and viral infection. Hemocyte effector functions are conserved across invertebrate taxa. What we learn in this study will provide important comparative information on the factors regulating anti-parasitic and anti-viral defenses in arthropods.